No mksquashfs progress bar if stdin is not a terminal

Explicitly documented updated conditional, as "-t" is not a
commonly seen shell test.

scripts/build/binary_rootfs

@@ -265,7 +265,10 @@ case "${LB_CHROOT_FILESYSTEM}" in
 		# Remove stale squashfs image
 		rm -f chroot/filesystem.squashfs
 
-		if [ "${_QUIET}" = "true" ]
+		# Do not display progress bar if:
+		# - Run with --quiet, or
+		# - stdin is not a terminal (e.g. in CI, cron, etc)
+		if [ "${_QUIET}" = "true" ] || [ ! -t 0 ]
 		then
 			MKSQUASHFS_OPTIONS="-no-progress ${MKSQUASHFS_OPTIONS}"
 		fi